Industrial ovens are often used to heat substances that yield gases capable of explosive combustion if sufficiently concentrated. To avoid the dangers of such combustion, these ovens have venting systems to purge the oven chamber during the heating cycle. Industry standards as promulgated by the National Fire Prevention Association identify two methods for safely using such vented ovens.
By the first method, if the operator does not known the concentration level of explosive gases in the oven chamber, he must frequently purge the oven chamber. Although the operator will never be sure of the actual concentration level, he can be reasonably assured that an explosive concentration cannot accumulate.
As an alternative, the operator may analyze the exhaust gases from the oven and determine the actual concentration level of the explosive gases. An analyzer for doing this may be found in U.S. Pat. No. 4,116,612. By relying upon such analysis, the operator need only purge the oven chamber upon reaching a maximum acceptable concentration level. This technique has many benefits over the first method, since frequently purging the oven chamber requires a significantly higher expenditure of energy to continually reheat the newly introduced oven atmosphere.
Gas analyzers generally include a sensor to sense the concentration level of a particular gas or gases. These analyzers are generally calibrated by establishing a zero condition and an upper acceptable limit. To establish this upper acceptable limit, the operator exposes the sensor to a known concentration of calibration gas called span calibration gas. Once calibrated, the analyzer will only take affirmative action when concentrations exceed this upper acceptable limit.
If the analyzer should become uncalibrated and accept a higher concentration of explosive gases without warning or appropriate action, a dangerous condition may arise. Because of this, some prior art devices provide for mechanisms whereby the analyzer may be occasionally recalibrated by reintroducing both zero calibration gases and span calibration gases. In general, these devices have an in-line solenoid valve between the oven chamber and the analyzer such that the flow of test gases to the analyzer may be shut off. Then, other valves will be opened to allow the zero or span calibration gases to be injected into the analyzer. Certain problems, however, have arisen as a result of these designs.
First, such mechanisms do not necessarily insure accurate calibration. In general, the analyzer operates by sucking exhaust gases into its sensor chamber by means of a vacuum pump. By injecting the zero or span calibration gases into the sensor chamber, however, the calibration gases will be introduced at a pressure higher than normal operating pressures. This difference may be significant, since an analyzer calibrated for a certain concentration level of gases delivered at one pressure may not properly respond to that same concentration level delivered at a different pressure. Consequently, the analyzer may accept a dangerously high concentration level even though apparently calibrated.
Second, using a solenoid valve to close off the test gas line to the analyzer has often been unsatisfactory. The test gases from the oven chamber will often include various silicones, resins, plasticizers, fillers and varnishes. These materials may collect in the solenoid valve orifice and cause the valve to become partially plugged. This becomes particularly troublesome when the temperature of the test gases exceeds the temperature rating for the solenoid valve, as will often be the case. Under such conditions, the solenoid value must be maintained at a lower temperature than the test gases, and this promotes condensation of the test gas material within the solenoid valve, thereby worsening the plugged condition. This may again result in a dangerous condition since the analyzer, even if properly calibrated, may not be receiving a sufficient sample quantity to make an accurate reading.
Therefore, a need exists for a combustible gas analyzer calibration apparatus that does not use solenoid valves in the test gas flow line to the analyzer, and for an apparatus wherein calibration gases will be introduced into the analyzer at substantially the same pressure as the test gases.